The present embodiments relate to multiplanar reformatting (MPR). MPR is used in medical visualization for extracting an arbitrarily oriented two-dimensional (2D) composite image. The MPR may be composed from a single image or a plurality of images representing intersecting planes in different positions and orientations within a single or a plurality of volumes. Typically, when an MPR is visualized using a plurality of reformatting planes, known as thick MPR, the planes have the same orientation but different offset positions (i.e., adjacent parallel planes). The data corresponding to the planes are composited into a single output image. The composition is by averaging, computing the maximum, or blending. FIG. 1 shows one example of compositing in MPR. For compositing by averaging, the data for the individual MPR planes is extracted, the sum of all the data for the extracted MPR planes is computed and stored into an intermediate sum buffer, and the final output is averaged.
For scrolling MPR, the thick MPR location is changed only along the viewing normal or direction (see FIG. 1). Scrolling MPR is used to reveal the structure and progression of anatomical structures. A sequence of visualizations representing different depths within the volume along the viewing direction is generated. Fast interactivity for scrolling MPR is important not only from the point of efficiency but is also a highly desirable feature for assisting clinical evaluation. FIG. 2 shows the planes for thick MPR edgewise with a dashed line indicating the included planes. Slow scrolling, such as by the one plane step shown, may be used for fine adjustment or considered diagnosis. Rapid scrolling, such as by the five-plane step shown, may be used to more quickly identify a region of interest.
The number additions and subtracts may increase with greater scrolling speeds. The increase may slow performance of the processor rendering the thick MPR, causing a slow down in scrolling or distracting jumps in visualization.
Acceleration techniques available in the art of volume rendering such as data downsizing, space leaping, and early ray termination may only accelerate thick MPR rendering for a limited number of cases. Data downsizing reduces the data used, but may decrease resolution. Data downsizing is not an acceptable practice for scroll MPR rendering since high resolution is desired for close clinical examination of pathologies. Space leaping and early ray termination techniques rely on intelligent skipping of image regions not visible in the rendering settings. However, particularly for average compositing, since all MPR values are needed in the averaging composition, any skipped image value may affect the output quantitatively. For maximum composition, space leaping techniques may be acceptable in some limited cases for improving interactivity and are not used for evaluation quality MPRs. In general, space leaping and early ray termination techniques are effective mostly for blend compositions. These volume rendering acceleration techniques alone cannot fully resolve the performance issues with diagnostic quality scrolling thick MPR.
Another method for accelerating scrolling thick MPR does so by enforcing a fixed scrolling increment rule. Scrolling is limited to fixed sampling distances. Under this rule, the planes within a thick MPR between the sequential visualizations usually highly overlap. If the data for the planes within the thick MPR is stored, the data may be reused without re-computation for the next visualization (thick MPR). FIG. 2 shows one example. This method is effective in reducing the MPR computation, but the memory storage requirement is usually too high when the thickness is large, such as tens or hundreds of planes.
Average composition may be assisted by storing a single sum from the previous visualization rather than the data for all the planes. In this scheme, at each scroll increment, the data for the removed plane image is recomputed and subtracted from the previous sum buffer. The data for the newly included plane is computed and added before averaging. For each visualization, the data for one or more planes no longer included in the thick MPR are subtracted and the data for planes now included is added to the sum. This method is highly effective for accelerating scrolling thick MPR when the number of scroll increments (i.e., the step size for scrolling) between visualizations is very small. For larger scrolling increments, little reduction in computations may result.
The control of the size of scroll increments is typically driven by the speed of a computer mouse, with fast speed mapping to stepping by a large number of planes. When the scroll increment is large, however, the simple fixed scrolling increment method fails to provide any acceleration, and in some cases actually induces extra computational overhead because the computation of the subtracted and added planes is intensive. The result is an unpleasant delay during scrolling interaction. The delay may distract a clinician from properly evaluating the anatomical structures in the images.